1. Field of the Invention
The present invention is generally related to the use of non-capsulated mutants of normally encapsulated bacteria as a vaccine. In a particular application, the invention provides a low cost vaccine for pleuropneumonia and other diseases caused by encapsulated bacteria that produce exotoxins.
2. Description of the Prior Art
Vaccines are preparations used to prevent specific diseases in animals by inducing immunity. This is accomplished by exposing a patient to an antigen for a particular disease which, in turn, causes the immune system of the patient to produce large quantities of antibody. The presence of the antibody in the patient's blood protects the patient from a later attack by the disease causing agent. Vaccines may either be composed of subunits of the agent, or the live or killed agent itself. For example, poliomyelitis is typically controlled by either administering a live, attenuated oral poliovirus vaccine, which is common practice for treating children, or by administering a killed or inactivated poliovirus vaccine, which is the usual practice for treating adults since they generally have a higher risk of contracting polio from the live vaccine. If a live vaccine is to be used, its virulence must be attenuated in some way; otherwise, the vaccine will cause the disease it is intended to protect against.
A number of diseases are caused by encapsulated bacteria wherein the capsule, which is the gum-like layer of polysaccharide or polypeptide exterior to the cell wall of these bacteria, is required for pathogenesis. For example, pleuropneumonia, a serious respiratory disease affecting swine worldwide, is caused by Actinobacillus pleuropneumoniae, and it is has been determined that the serotype-specific antigen is the polysaccharide capsule (see, Inzana et al., Infection and Immunity, 55:1580-1587 (1987), which is herein incorporated by reference). In addition, in pneumonia of cattle caused by Pasteurella haemolytica, and respiratory disease of humans caused by encapsulated Pseudomonas aeruginosa, the polysaccharide capsules are also important in the virulence of these bacteria.